Dissolving pulp, also known as dissolving cellulose, is a bleached wood pulp that has a high cellulose content and is produced chemically from the wood by using a sulfite process or a kraft process. The kraft process is a commonly used pulping process and in a conventional kraft process, wood is treated with an aqueous mixture of sodium hydroxide and sodium sulfide. This treatment degrades and solubilizes lignin leading to a defibration of the wood fibers.
Furthermore, conventional manufacturing of dissolving pulps by kraft processes, such as kraft processes comprising a prehydrolysis step, suffer from low yields as the hemicelluloses in the wood are degraded during the process, mainly in the prehydrolysis step and are transferred into an acid condensate as low-molecular weight hemicellulose, monosaccharides and hemicellulose degradation products. Due to difficulties in extracting these degradation products from the digester, the degraded material is at best used for energy production by evaporation and burning of the components or else simply discarded as waste.
WO 99/47733 discloses a process for producing cellulosic fibers, wherein the degree of polymerization of the obtained fibers can be adjusted via acid hydrolytic and oxidative degradation. However, the kraft pulp obtained by this process has high amounts of residual hemicelluloses, which makes the obtained pulp less useful for the production of regenerated cellulose for use in e.g. textile applications as these residual compounds have a negative impact on the process behavior and, as a result thereof, also on the textile-mechanical properties of the fibers produced therefrom.
US 2009/0312536 discloses a process for producing dissolving pulp suitable for textile applications from a cellulosic starting material using a kraft process which has been combined with a purification step of cold caustic extraction (CCE) type. The processes disclosed in US 2009/0312536 are not optimized for implementation in a kraft pulp mill, i.e. in an industrial scale process.
Both WO2011/138633 and WO2011/138634 disclose methods for pulp processing including a cold caustic extraction step. However, the disclosed methods describe costly procedures having a low total yield of dissolving pulp.
Accordingly, it is an object of the present disclosure to provide an improved industrial scale process for producing high yield dissolving pulp in an efficient and economical manner.